This invention relates to an acid catalyst that contains a substantial amount of sulfated zirconia and at least one hydrogenating transition metal as well as to its uses in hydrocarbon transformation chemical reactions requiring the use of an acid type catalyst, such as for example those of isomerization, alkylation, oligomerization or yet dehydration reactions of light hydrocarbons, but also hydrocracking or hydroisomerization reactions of heavier hydrocarbons.
In the following, the term xe2x80x9csulfated zirconiaxe2x80x9d does not mean zirconium sulfate or zirconyle stoichiometric sulfate, but zirconium (zirconium dioxide) more or less sulfated, where the sulfate content can be less than that of the above-mentioned stoichiometric compounds.
As known, the oil industry uses many methods to modify hydrocarbon structures in order to obtain molecules whose properties are suitable for the sought use. These procedures usually call for one or more catalysts that have to be specifically adapted to the chemical transformation one wants to complete, as well as to the requirements tied to the implementation of the method.
Many of these hydrocarbon transformation chemical reactions are done with an acid type catalyst. Such is the case, for example, of reactions taking place in the isomerization process of paraffin, which applies mostly to light gasolines and allows for the transformation of linear paraffin into ramified paraffin, whose octane number is higher.
In this method, the acid catalysts most used today are catalysts with an aluminum chloride base supported on alumina (meaning deposited on an alumina support). Indeed, these extremely active catalysts make it possible to obtain an isomerization reaction at low temperatures, around 150xc2x0 C., with a thermodynamic balance that is very favorable to the formation of the sought products.
However, this type of catalyst does have a certain number of inconveniences tied in particular to the fragile nature of its active sites. Indeed, the aluminum chloride is a very unstable compound: it is irreversibly destroyed by water, oxygen, oxygen-containing or sulfur compounds. These products must therefore be entirely eliminated from the load being treated, which is quite costly and restricting. Furthermore, the loading of the reactors when starting the unit or, when the catalyst has been replaced, must be done in perfectly anhydrous conditions, without any trace of water or oxygen. Moreover, the preservation of the active sites during the operation requires a constant injection of dopants such as hydrochloric acid or other chlorine products; the excess acid must then be removed when leaving the reactor and invariably creates a corrosion problem. Lastly, despite all these precautions, the catalyst is progressively destroyed and must be replaced periodically since it is not regenerable.
This is why the research relating to acid catalysts has looked to the creation of new compounds with catalytic properties similar to those of the aluminum chloride but without having the same inconveniences as the latter. This is specifically the case of the sulfated zirconia.
Thus, U.S. Pat. No. 3,032,599 (Phillips Petroleum) is one of the first patents to describe the application of sulfated zirconia to isomerization and alkylation of hydrocarbons: the proposed catalysts are made entirely of zirconia gel, possibly containing small quantities of a metallic promoter. They are prepared by the precipitation of a zirconyle salt in solution in water, by an addition to the base. The zirconia gel obtained is then sulfated and then activated at approximately 500xc2x0 C. These catalysts do indeed show acid catalytic properties but, however, they are not very satisfactory. Indeed, they have a low surface area, which can explain their relatively mediocre performance as far as isomerization reactions are concerned. Furthermore, these powdery catalysts are for the most part unusable as such in an industrial reactor.
Also, U.S. Pat. No. 3,132,110 (Union Oil) describes the properties of a series of acid catalysts with a base of hydrated zirconia containing sulfate radicals, pure or preferably combined with alumina. The preparation methods of these catalysts rest mostly on the decomposition of a zirconia sulfate salt in solution in water, by the hydrolysis in a basic medium or by thermal decomposition. The catalysts obtained in this manner are indeed active in a good number of reactions requiring the use of an acid catalyst, and have the advantage of being perfectly regenerable. Nevertheless, their activity has proved to be relatively limited and these catalysts must be used at high temperatures, for example over 370xc2x0 C. in the case of the isomerization reaction of paraffin. Well, at such temperatures, not only is this reaction disadvantaged thermodynamically but, in addition, the catalyst""s deactivation speed is accelerated by depositing coke on its surface.
Along the same lines of replacing aluminum chloride based catalysts in isomerization with more stable catalytic compounds, U.S. Pat. No. 4,406,821 (Exxon) proposes a catalyst consisting of a sulfated oxide deposited on an alumina support. This oxide is preferably a tungsten or hafnium oxide, but can also be a niobium, thallium, zirconium oxide, or a mixture thereof. This catalyst is prepared by impregnation of the alumina support with a solution of a salt of the chosen metal, followed by a calcination at a high temperature, then a sulfation using a sulfuric acid water solution. The catalyst obtained by this method does possess the acid properties and it performs particularly well in the etherification reactions of phenols. Nevertheless, these catalysts are not well adapted to the isomerization reaction of paraffin at low temperatures, where their activity seems to be limited.
In general, the acid catalysts proposed in the prior art and which could replace the alumina chloride based isomerization catalysts are therefore quite unsatisfactory due to their lack of activity.
Continuing her research in the field of sulfated zirconia based catalysts, the petitioner has issued the hypothesis that the lack of activity of the formulas proposed to date was tied to the actual structure of these catalysts that do not present enough accessible active sites to the reactive molecules. She has deduced that this was due to a maladjusted porosity and to a catalyst surface area too small in the prior art""s catalysts, and an incapacity to control these parameters.
This is why the petitioner has focused her efforts on the problem, at the time unsolved, of improving the exchanges between the active catalyst sites and the molecules to be converted. She assumed it was necessary to succeed in modifying the structure of these sulfated zirconia based catalysts, and she then focused on creating catalysts that have a more adequate porosity and surface area, appropriate for giving them a better activity compared to what has been accomplished to date. In doing so, she has also discovered a certain number of original methods for controlling the porosity of these catalysts and shaping them so as to obtain the desired active structures.
So, the applicant has perfected a solid acid catalyst, containing a substantial quantity of supported or mass sulfated zirconia and at least one hydrogenating transition metal. This catalyst is different in that the said sulfated zirconia is in a crystallized form and shows a surface area greater than or equal to 135 m2/g, preferably greater than or equal to 150 m2/g, a pore volume greater than or equal to 0.16 cm3/g, preferably greater than or equal to 0.2 cm3/g, and more preferably greater than or equal to 0.25 cm3/g and an average pore diameter greater than or equal to 20 Angstroms (20xc3x9710xe2x88x9210 m).
Here and in the following, the characteristics of the surface area, the pore volume and the average pore diameter are mentioned in reference to the method of determination called B.E.T. (Brunauer, Emmett, Teller) by adsorption of nitrogen, well known to any person skilled in the art, as described in the work by S. Lowell and J. E. Shields, xe2x80x9cPowder surface Area and Porosityxe2x80x9d, Powder Technology Series (1984). The surface area S is deducted from the B.E.T. linear transformation at five points, the pore volume Vp is determined according to the quantity of nitrogen absorbed in a relative pressure P/Po=0.985 and the average pore diameter Rp is calculated following the formula Rp 2Vp/S.
In the catalyst consistent with the present invention, the zirconia (zirconium dioxide) is partially or totally sulfated. Favorably, the sulfate content is less than the stoichiometric quantities: preferably, the sulfur content in form of sulfate is between 1% and 10% by weight compared to the weight of the zirconia, and even more preferably, between 1% and 5% by weight.
According to the invention, the structure and texture characteristics that define the catalyst have been optimized by acting on the manufacturing method of these catalysts: by resorting to adequate methods such as the deposit of sulfated zirconia on an appropriate support or the potential use of a structuring agent, by acting both on the nature and the sequence of the manufacturing stages of these catalysts, by implementing appropriate thermal treatments (in particular, calcinations), it has been possible to modify the structure and the texture of sulfated zirconia based catalysts in order to select the most active formulas.
In a preferred embodiment of the method of the present invention, a hydrated zirconia gel is washed with a soluble polar organic solvent (other than water). In accordance with the method including this washing step, a solid acid catalyst containing pure mass sulfated zirconia in a crystallized form has a surface area greater than or equal to 150 m2/g, a pore volume greater than or equal to 0.2 cm3/g, and more preferably greater than or equal to 0.25 cm3/g, and an average pore diameter greater than or equal to 20 Angstroms (20xc3x9710xe2x88x9210 m) and preferably greater than or equal to 30 Angstroms (30xc3x9710xe2x88x9210 m).
When compared to the sulfated zirconia based catalysts known to date, the catalyst according to the invention shows a crystalline structure that helps give it a more open porosity, and a higher specific surface area. This generates a better ease of access for the reactants to active sites that are themselves more numerous, which, in turn, gives the catalyst an increased activity: for the isomerization reaction of linear paraffin, it has been proved to possess an activity close to that of traditional alumina chloride based catalysts. As the latter, the invention""s catalyst stays active at low temperatures (approximately 150xc2x0 C.), thus in conditions for which the isomerization reaction of linear paraffins is thermodynamically favorable to the sought ramified products.
Furthermore, even though the invention""s catalyst has catalytic properties similar to those of traditional aluminum chloride catalysts, it does not have the same disadvantages: much more stable, it has indeed proved to be less sensitive to the inevitable presence of small quantities of water and sulfur compounds in the loads to be converted. Indeed, the water does not act on the catalyst""s active sites in a destructive manner but in an inhibitory manner, and thus in a reversible manner, since this catalyst can easily be regenerated. In the same manner, there is no need for any specific precautions to be taken when storing and loading this catalyst.
Also, the invention""s catalyst has the undeniable advantage of being perfectly regenerable, by the combustion of coke deposits created during the isomerization reaction. This property is particularly interesting from an economic point of view considering the high cost of catalysts. The regeneration can be handled advantageously on site, meaning without having to remove the catalyst from the reactor, which avoids the multiplication of unloading and loading functions of the latter.
It is then possible to consider a continuous catalyst regeneration method, where the catalyst circulates between a reactor in which the conversion takes place and a catalyst regeneration chamber. Such a method avoids having to periodically stop the unit in order to regenerate the catalyst and more importantly to have, in the reactor, a catalyst permanently kept at the height of its activity.
Furthermore, the use of the invention""s catalyst does not require the injection of corrosive dopants in the reactor. As a result, the corrosion is reduced within the unit which in turn allows for a longer life time of the unit. It also results in an enhancement of the security tied to the method, and in particular to a reduction of pollution and accident risks connected with the use of such dopants, especially the toxic dopants.
Lastly, the invention""s catalyst has proved to be remarkably active in a certain number of reactions other than the isomerization of light paraffin. Among others, the alkylation reactions, the dehydration reactions, and mostly the hydrocracking and hydroisomerization reactions of longer light paraffins (consisting of more than 7 carbon atoms).
Concurrently, the petitioner have perfected several original methods allowing for the control of both the structure and the texture of the sulfated zirconia based catalysts, and therefore for the preparation of a catalyst with a controlled porosity as described previously. Thus the invention also relates to these preparation procedures, which will be explained in more detail in the description and the examples hereafter.
In order for the catalyst to be sufficiently active, the sulfated zirconia that it contains must be in a crystallized form, meaning it must not be in an amorphous form. The preferred crystallized structures are the quadratic and monoclinic type structures. The determination of the crystalline structure of the zirconia is created, in a well-known manner, by x-ray diffraction.
The presence of a hydrogenating transition metal is necessary for the stability of the catalytic activity of the invention""s catalyst. This hydrogenating transition metal is preferably an element pertaining to group VIII of the Periodic table of elements, and in particular an element of the group consisting of platinum, ruthenium, rhodium, palladium, osmium and iridium, where the platinum is most preferred. The crystalline structure of the zirconia is not affected in a sensitive manner by the presence of this hydrogenating transition metal.
The acid catalyst, according to the invention, is of the solid type. It can exist under all forms to which the trade person usually resorts for the implementation of solid catalysts, and in particular in the form of particles such as beads, extrusions, and pellets. It has a visible filling density preferably between 0.5 and 3.
Preferably, its surface area is greater than or equal to 150 m2/g, its pore volume is greater than or equal to 0.25 cm3/g and its average pore diameter is greater than or equal to 30 Angstroms (30xc3x9710xe2x88x9210 m).
The crystallized sulfated zirconia present in the invention""s catalyst can have two distinct forms: one called xe2x80x9csupportedxe2x80x9d or another called xe2x80x9cmassxe2x80x9d.
In the case of a crystallized sulfated based zirconia in the xe2x80x9csupportedxe2x80x9d form, the sulfated zirconia crystals are deposited on a support that has already formed. The latter can be any kind of support usually used for industrial catalysts, such as alumina, silica, silica-alumina, silicate, alumino-silicate, magnesia, zeolite, active carbon, gallium, titanium, thorium, or boron oxide, clay and any combination of these supports. Preferably, the invention""s catalyst then contains between 50% and 95% by weight of classic support, on which 5% to 50% of sulfated zirconia by weight is deposited.
In the case of a crystallized sulfated zirconia based catalyst in xe2x80x9cmassxe2x80x9d form, such catalyst contains zirconia crystals in its matrix, alone or mixed with a structuring agent, which allows for a better control of the catalyst""s structure and texture. This structuring agent can be any refractory mineral oxide usually used in the industry, and can in particular be chosen from the group consisting of aluminas, silicas, silica-aluminas, alumino-silicates, clays and combinations of these compounds. The catalyst can then contain from 0.5 to 100 percent by weight of the sulfated zirconia and preferably from 20 to 80 percent by weight.
In the case of the crystallized sulfated zirconia based catalyst in xe2x80x9cpure massxe2x80x9d form, such catalyst contains zirconia crystals in its matrix, alone or mixed with a binder material, whose only function is to facilitate the shaping of the catalyst, considering the powdery characteristic of the zirconia. This binder material can be any refractory mineral oxide usually used in the industry for that purpose, and can in particular be chosen from the group consisting of aluminas, silicas, silica-aluminas, alumino-silicates, clays and combinations of these compounds. The catalyst can then contain from 5 to 50 percent by weight and preferably from 10 to 30 percent by weight of the binder material.
The invention""s acid catalyst can be prepared according to a certain number of methods detailed hereafter. These methods make it possible to have a strong control of the textural and structural properties of the catalyst, which is indispensable in preparing a catalyst that satisfies the characteristics of the invention, thus a highly active and usable catalyst as is in an industrial reactor.
a) Supported Sulfated Zirconia Based Catalyst
The solid acid catalyst, with a sulfated zirconia base, deposited on a catalytic support can be prepared according to a method consisting of the following steps:
deposit of hydrated zirconia on the catalytic support
calcination of the solid
sulfation of the solid
deposit of a hydrogenating transition metal
final calcination of the solid.
The deposit of the hydrated zirconia on the catalytic support can be done by impregnation of the mentioned support through a solution of zirconium salt followed by drying the solid obtained.
The deposit of a hydrogenating transition metal on the catalytic support can be done before the deposit of zirconia or at any other time during the preparation but must be done prior to the final calcination.
b) Mass Sulfated Zirconia Based Catalyst
In these catalysts, the matrix has a base of zirconia, pure or mixed with a structuring agent containing a refractory mineral oxide base or a mixture of refractory mineral oxides.
The solid acid catalyst, with a mass sulfated zirconia base mixed with a structuring refractory mineral oxide, can be prepared according to a method consisting of the following steps:
addition of a basic solution to a solution of zirconium salt, so as to produce a precipitation of hydrated zirconia,
addition of salt precursor solution to the structuring refractory mineral oxide
addition of a basic solution, so as to produce a precipitation of the structuring refractory mineral oxide
washing then drying of the product obtained
shaping of the solid
sulfation of the solid
deposit of the hydrogenating transition metal
final calcination
The precipitation of the zirconia can be done before or after that of the structuring agent. The steps consisting of the deposit of the hydrogenating transition metal and the shaping can be done before or after the sulfation but must be done prior to the final calcination.
According to a first alternative, the solid acid catalyst, with a mass sulfated zirconia base mixed with a structuring refractory mineral oxide, can be prepared according to another method consisting of the following steps:
addition of a basic solution to a solution of zirconium salt and of a salt precursor to the structuring refractory mineral oxide, so as to produce a co-precipitation of zirconia and of the structuring refractory mineral oxide.
washing, drying of the precipitate obtained
shaping of the solid
sulfation of the solid
deposit of the hydrogenating transition metal
final calcination
The steps consisting of the deposit of the hydrogenating transition metal and the shaping can be done before or after the sulfation but must be done prior to the final calcination.
The solid acid catalyst, with a pure mass sulfated zirconia base can be prepared according to a method consisting of the following steps:
addition of a basic solution to a solution of zirconium salt, so as to produce a precipitation of hydrated zirconia
washing then drying of the precipitate obtained
sulfation of the solid
shaping of the solid
deposit of the hydrogenating transition metal
final calcination
The steps consisting of the deposit of the hydrogenating transition metal and shaping can be done before or after the sulfation, but must be done prior to the final calcination.
The solid acid catalyst according to the present invention, with a crystallized xe2x80x9cpure massxe2x80x9d sulfated zirconia base, can be prepared according to one embodiment of the invention by a method comprising the steps of:
a) adding a basic solution to a zirconium salt solution to produce a mixture containing a precipitate of hydrated zirconia,
b) filtering the mixture and washing the precipitate with water to form a solid,
c) washing the solid with a water soluble polar organic solvent, at least one time,
d) drying the solid,
e) sulfating the solid,
f) shaping the solid and first calcining at a temperature of at least 550xc2x0 C.,
g) depositing the hydrogenating transition metal, and
h) final calcining.
The steps consisting of the deposit of the hydrogenating transition metal and shaping can be done before or after the sulfation, but must be done prior to the final calcination. The solid is dried only after the step of washing with a water soluble polar organic solvent. Preferably, the solubility in water of this solvent is of at least 5 g/100 ml. In other words, at least 5 g of solvent may be dissolved in 100 ml of water at 23xc2x0 C., under atmospheric pressure (760 mm Hg). More preferably, the solvent and water are miscible in all proportions. The boiling point of the solvent is preferably less than the one of water (i.e. less than 100xc2x0 C.). During the washing step, the gel is washed at least once and preferably several times (at least three times). After drying the zirconia gel treated in this way and sulfating the same, the resulting product may be crystallized by subjecting it to a calcination and, surprisingly, still keep high textural properties, in particular a specific surface area or 150 m2/g or more, a pore volume of at least 0.2 cm3/g and an average pore radius of at least 20 xc3x85.
It should be noted that, when shaping this type of catalyst, usually a binder material is preferably incorporated. This binder material makes it easier to shape the catalyst into extrudates or other pellets, to be used in an industrial reactor. This binder is not a structuring agent. The catalyst comprises between 5 to 50 wt % of this binder and preferably 10 to 30 wt %. The binder may be selected from the group consisting of aluminas, silicas, silica-aluminas, alumino-silicates, clays and combinations thereof.
No matter what type of catalyst is being prepared or what method of preparation is being used therefore:
the zirconium salt to be used in the process of the invention can be selected from the group consisting of nitrates, chlorides, acetates, formates, oxalates of zirconium and zirconyles as well as zirconium propylates and butylates
the salts that can be used as precursors of the structuring refractory mineral oxide, if necessary, are well known to the person skilled in the art. If for example, the aforementioned mineral oxide is an alumina, we will use an aluminum salt that can favorably be selected from the group consisting of nitrates, chlorides and aluminum sulfates.
the basic solution used can be any-solution that will help achieve the precipitation of a hydrated oxide from a solution of salt precursor of said oxide by increasing the pH. For example, it can be that of an ammonia solution or any other base known to the person skilled in the art.
the step consisting of the sulfation of the catalyst is done by impregnating the solid with a sulfating agent, then drying. The sulfating agent can be liquid, gaseous or in solution; for example the following can be used, a sulfuric acid pure or in solution, an aqueous solution of ammonium sulfate, or any other precursor of sulfate ions. To achieve this sulfation, any impregnation technique known to the person skilled in the art can be used. This step usually ends by a calcination of the sulfated solid.
the step consisting of the shaping of the invention""s solid catalyst, necessary when it contains mass zirconia, makes it possible to clump the catalyst powder in the form of particles (for example beads, extrusions or pellets) in order to be able to directly use this catalyst in an industrial reactor. To facilitate this operation, it may be necessary to add a binding material (alumina xerogel or any other industrial binding material) to the catalyst powder, then to knead the mixture obtained before proceeding with the actual shaping by extrusion, xe2x80x9coil dropxe2x80x9d, container method, or any other method known for the shaping of industrial catalysts. This step ends with a calcination.
the step consisting of the deposit of the hydrogenating transition metal is achieved by impregnation of the solid with a solution of hydrogenating transition metal compound, followed by a drying step; when this metal is platinum, the impregnation step is carried out with a solution of a platinum compound that can be selected from the group consisting of chloroplatinic acid and complex platinum compounds.
the first calcination, usually completed after the shaping, must take place at a temperature that is sufficiently high, that is to say greater than or equal to 550xc2x0 C. This is necessary in order to obtain a zirconia with an adequate crystalline structure.
The preparation methods described above are only suggestions for the preparation of a catalyst that is consistent with the invention. Of course, they have no restricted character. If necessary, the person skilled in the art will know exactly how to adapt them through additional well-known operations such as, for example, the ripening of gels, washing with solvents, drying and calcination.
The invention""s acid catalyst can be used in any hydrocarbon transformation chemical reaction requiring the use of an acid, or super acid, catalyst.
This catalyst has proved to be especially advantageous for isomerization reactions of linear paraffin into ramified paraffin at a temperature below 200xc2x0 C., but it can just as well be used in the isomerization of olefins and the isomerization of cyclical and aromatic compounds. It can also be used in an alkylation reaction, an oligomerization reaction or a hydrocarbon dehydration reaction.
Furthermore it can be used, quite advantageously, in a treatment method of a hydrocarbonic section containing a substantial quantity of long chain paraffin, whether linear or slightly ramified, such as for example paraffin stemming from a Fischer-Tropsch type synthesis (hydrocarbon synthesis from the CO+H2 mixture). The transformation by hydrocracking or by hydroisomerization of this paraffin is often necessary in order to obtain either xe2x80x9clargexe2x80x9d products (medium gasolines, naphtas, or distillates) or specialties (high quality paraffin or lubricants). The operating conditions must then be adjusted in relation to the reaction that is favored (hydrocracking or hydroisomerization) and to the desired level of conversion. Preferably they will be as follows: a temperature between 20xc2x0 C. and 200xc2x0 C. (preferably between 50xc2x0 C. and 150xc2x0 C.), a pressure between 5xc3x97105 and 100xc3x97105 Pa (preferably between 20xc3x97105 and 60xc3x97105 Pa), a molecular hydrogen/hydrocarbon H2/HC ratio to be converted between 1 and 20 (preferably between 5 and 15).
The invention""s catalyst can be stored or loaded in a reactor without taking any specific precautions. It is however preferable to submit it to a calcination at high temperature in a dry atmosphere before using it.
After being used, it can be regenerated by simply passing it through an oxidizing atmosphere at a temperature in the range of 400 to 700xc2x0 C.